In spite of approximately 50 morphological and physiological criteria currently used in yeast systematics, DNA base composition studies have shown a wide intraspecific variation in moles per cent guanine plus cytosine (%GC) in the nuclear DNA of a number of presently accepted yeast "species". Conversely, the indiscriminate use of a large number of such criteria, has led to the description of many new "species" sometimes differing by only a single biochemical ability, for example hydrolysis of a disaccharide. In several of the latter cases we have shown high degrees of DNA-DNA homology or renaturation ability between species separated on this basis. Such "species" may differ by only a single gene or even by a single base pair in the case of point mutation We are continuing to study strains of yeast species to determine variations in DNA base composition. As an example, a high proportion of the yeasts associated with necrotic tissue or cacti in the Sonoran desert were thought to be strains of Pichia membranaefaciens on the basis of current taxonomic criteria. On the basis of GC analyses we have shown that none of these strains represent P. membranaefaciens but that they consist of five novel separate species. New taxonomic parameters have been found to distinguish these species. We are also conducting DNA/DNA reannealing experiments between nuclear DNAs of what appear to be closely related species with similar base composition. For these experiments we used DNA labelled in vitro with 125I. The duplexes are differentiated from unreacted single-stranded DNA by adsorption on hydroxylapatite or by enzymic digestion of ss DNA. The results are expressed in percent relative binding of heterologous versus homologous DNA. The species under study belong to the genera Kluyveromyces, Hansenula and Pichia.